My impression is the rear amp and woofer in my car are dead since years. I do not have any subwoofer kick in my music from what I can tell. The music plays quieter in the rear than in the front.

 

Did you try the balance, and fader controls?

 

Of course. There is just something gone in the rear since many years now. What if the rear amp goes dead, is there an automatic bypass allowing the head to take control of the speakers?

 

No automatic bypass. Like was said before, most likely bad speakers. Although the sub doesn't fail the same as the door speakers.

There's something wrong with my headunit. When using an aux cable(with my phone), only the right front speaker works. When I also plug in the charger cable for the phone, it gets a little louder, but still only the one speaker. I've tried several different aux cables.

Not threadjacking, just pointing out that the problem could be in the headunit or CD player.

 

Go thru the Crutchfield site pick out the HU, speakers , sub woofer, Amp and tweeters you like or can afford and upgrade, maybe add in a touch screen with a backup camera, or not.

 

I too missed something.
Started out as a CD player problem and no mention of the door or picture of it.

 

Reread posts #13-17. The answer is there.

 

I see Randy talks about the door thing for his car but I don't see any pictures.
But no matter, move on and see if the OP problem is fixed.